In recent years, in the field of liquid crystal displays, most notably in TV applications, there has been increasing demand for displays with larger screens, higher luminous intensities, smaller thicknesses, and better luminance uniformity. For this reason, direct-type backlight systems have been used, which comprise in combination a light source using multiple cold-cathode tubes, a reflector provided in the backside, and a diffuser plate constituting a luminescent surface. While such a device has a high effective utilization factor of the light flux radiated from the light source (the ratio of the light flux radiated from the luminescent surface to that radiated from the lamp) and makes it possible to increase the number of light sources to be used thereby facilitating the enhancement of the intensity of the luminescent surface, it has a problem in that the luminance uniformity of the luminescent surface is degraded since the luminous intensity is increased just above the light source. This problem would be pronounced especially when the thickness of the backlight device is reduced.
Conventionally, in order to obtain a backlight device which combines two contradictory features; effective utilization of light and elimination of the lamp image from the light source, a method for obtaining a diffuser plate to be used therefor has been investigated in which inorganic particulates or cross-linked organic particulates are mixed as the light diffusing agent with substrate resin, for example, methacrylate resins, polycarbonate resins, styrene resins, and vinyl chloride resins (see patent document 1).
For example, from a viewpoint of transparency and resistance to ultra-violet rays, method of using methacrylate resins as the substrate resin has been investigated (see, patent document 2). However, a diffuser plate made of methacrylate resin has the problem in that it is likely to be subject to dimensional changes and warping due to moisture absorption. The space formed by the light source, the reflector, and the diffuser plate tends to have a smaller volume per unit display area since the distance between the reflector and the diffuser plate tends to be smaller, for example, not more than 15 mm due to the demand for thinner liquid crystal displays in recent years. As the result, the temperature rise curve after turning on the light source tends to be increasingly steeper; for example, the temperature at the light-source side of the diffuser plate tends to reach not lower than 50° C. within one hour after the light source is turned on, thereby more significantly affecting the warping and deflection of the diffuser plate. Since, in a backlight device, multiple sheets of various optical films such as a diffusion sheet may be disposed on the top surface of the diffuser plate, warping or deflection of the light diffuser plate may cause warping or deflection of the various optical films disposed on the top of the light diffuser plate eventually causing wrinkles thereon. As the result, in the region where wrinkles are generated, a problem arises in the form of a defect on the display surface. Moreover, unstable dimensions of the light diffuser plate will not only impede the assembly of the backlight device, but also negatively affect the product quality after manufacture.
On the other hand, from a viewpoint of preventing the occurrence of warp and deflection, there have been proposed methods utilizing a substrate resin which is less moisture absorbent than methacrylate resins, including a method utilizing polycarbonate resins (for example, see patent document 3) and a method utilizing styrene resins (for example, see patent document 4).
The luminescence principle of the cold cathode tube used in a direct-type backlight device is as follows: in a tube within which rare gas such as argon is enclosed in addition to mercury to improve the luminous efficiency, (i) electrons are released from the cathode by a high electric field applied to the electrodes, (ii) the electrons are accelerated by the high electric field to collide with mercury atoms thereby exciting the mercury atom, (iii) since the excited mercury atoms are unstable, they quickly return to a stable condition, releasing excess energy thereof in the form of ultraviolet radiation (mostly of 253.7 nm), and (iv) fluorescent substance absorbs this ultraviolet radiation and gets excited to emit light by transforming the ultraviolet radiation into visible light. It is known that a lamp, consequently, emits ultraviolet radiation of 254 nm, which is emitted by mercury and is essentially unnecessary, and other ultraviolet radiations such as of 365 nm (for example, see non-patent document 1). As has been described so far, as the number of fluorescent tubes increases and the luminous intensity per single tube increases from the demand for higher luminous intensity, increasingly larger amount of ultraviolet radiation tend to be emitted. In addition to that, as the distance between the diffuser plate and the light source is reduced from the demand for thickness reduction, and the ultraviolet energy illuminating the diffuser plate just above the tube tends to increase further, higher resistance to ultraviolet rays is demanded more than ever for the light diffuser plate to be used under such light sources.
[Patent Document 1] JP,A,54-155244.
[Patent Document 2] JP,A,01-172801.
[Patent Document 3] JP,A,03-143950.
[Patent Document 4] JP,A,06-345925.
[Non-patent Document] NIKKEI MICRODEVICE, p 73, June 2003, Published by NIKKEI BP Co., Ltd.